Crystal structure and characterization of a new one-dimensional copper(II) coordination polymer containing a 4-aminobenzoic acid ligand

A new coordination polymer based on CuII and 4-aminobenzoic acid is isolated and characterized using single-crystal X-ray diffraction, FTIR and fluorescence spectroscopy, and thermal analysis.


Chemical context
Coordination polymers (CPs), which can be categorized in the class of lower dimensional metal-organic frameworks (MOFs), have received great attention in the past few decades owing to the multitude of applications they offer, such as gas storage and separation (Fe ´rey, 2008), sensing (Horcajada et al., 2012), drug delivery (Liu et al., 2020), electrochemical applications (Morozan & Jaouen, 2012), adsorption and remediation (Baruah, 2022), magnetic properties (Maspoch et al., 2004), etc.Despite advancements, the anticipation of MOF structures remains an ongoing challenge.Even with reticular synthesis initiated by geometrically analogous ligands, the outcome of structures or ligand behaviors under elevated temperature and pressure conditions, prevalent during synthesis, remains complicated (Szczypin ´ski et al., 2021).Occasionally, in the pursuit of creating porous architectures, our efforts yield coordination polymers with unexpected features.In the present work, we attempted to synthesize a porous metal-organic framework based on Cu II and a flexible tricarboxylic acid ligand, 4,4 0 ,4 00 -{[(1E,1 0 E,1 00 E)-benzene-1,3,5triyltris(methaneylylidene)] tris(azaneylylidene)}tribenzoic acid (H 3 bttta) (Fig. 1).Instead, we obtained a one-dimensional CP, {[Cu(pABA) 2 (H 2 O)]•H 2 O} n (I), with the anion of p-aminobenzoic acid (pABAH), the latter presumably formed by disintegration of H 3 bttta in the course of hydrothermal synthesis.Subsequently we synthesized compound (I) from Cu(NO 3 ) 2 •2.5H 2 O and pABAH under the same synthetic conditions.Compound (I) was characterized by single-crystal X-ray diffraction, FTIR spectroscopy and thermogravimetric analysis (TGA).
Its anion, pABA, is capable of versatile binding with metal ions via amino and carboxylic groups (Fig. 2), as well as strong hydrogen bonds and �-� stacking interactions, enhancing the overall stability of the CP.Moreover, pABAH has a variety of applications, viz.as precursor in the synthesis of pharmaceutical compounds, UV absorbers, components in hair dyes, antioxidants, food additives, etc.

Structural commentary
Compound (I) crystallizes in a monoclinic space group P2/c, although the unit cell is metrically orthorhombic.The asymmetric unit comprises half of Cu atom, one pABA ligand and one water molecule.The Cu atom is disordered between two alternative sites, Cu1 and Cu2, both located on crystallographic twofold axes, with crystallographic occupancies of 0.3098 (8) and 0.1902 (8), respectively.The carboxylic group is also disordered, the atomic sites C1A and O1A are occupied simultaneously with Cu1 and have occupancies of 0.6196 ( 16), whereas C1B and O1B are occupied simultaneously with Cu2 and have occupancies of 0.3804 (16).The H atoms of the amino group are also disordered between two sets of positions with the same occupancies, depending on whether the adja-cent Cu1 or Cu2 site is occupied and coordinated with N1.The disorder is illustrated in Fig. 3.
It is noteworthy that the atomic positions (including those of the disordered atoms) approximately comply with the orthorhombic symmetry (apparent space group Pbcm), but their occupancies do not, therefore refinement of the structure in this symmetry gives a computationally unstable, as well as chemically and crystallographically unreasonable, model.
Both the Cu1 and Cu2 sites have an N 2 O 3 square-pyramidal coordination environment, in which the apical position is occupied by an aqua ligand (i.e. the O3 or O4 atom, respectively), also located on a twofold axis.Note that the water sites, unlike the Cu ones, are fully occupied.Thus, if the Cu1 site is occupied and Cu2 is vacant, O3H 2 is an aqua ligand and O4H 2 is a water molecule of crystallization and vice versa if the Cu2 site is occupied.
The pABA ligand bridges two adjacent Cu atoms (related by the c glide plane) through amine nitrogen and carboxylate oxygen atoms in a � 2 -O:N binding mode.Thus each Cu atom is linked with two symmetry-equivalent ones by pairs of antiparallel pABA ligands (whose two O and two N atoms comprise the basal plane of the pyramid), to form a polymeric chain parallel to the c axis.

Supramolecular features
The one-dimensional catena-Cu(pABA) chains of (I) are combined into a three-dimensional supramolecular structure by a network of hydrogen bonds (Table 1).Both water molecules (whether coordinated or not) donate hydrogen bonds to the non-coordinated carboxylic atom O2 (and its equivalents), forming an infinite zigzag chain O2� � �H-O3-H� � �O2� � �H-O4-H� � �O2 along the a-axis direction.The amino group, which is disordered over two orientations (see above), in either case donates one hydrogen bond to a trans-annular O2 and the other to the water molecule, which is not coordinated (the adjacent Cu site being vacant).Thus, while an aqua ligand donates two hydrogen bonds, the crystallization water at the same site donates two and accepts two, from different adjacent Cu(pABA) chains.

Spectroscopic and thermal properties
The FTIR spectra of pABAH and (I) (Fig. 4) demonstrated successful incorporation of the pABA ligand in (I).In comparison to the free ligand, pABAH, the peaks corresponding to the amine group suffer a decrease in the wavenumber and intensity upon binding to the Cu II atom in (I), similar to what is observed in other cases in the literature (Crisan et al., 2019).In addition, the peak at 1661 cm À 1 , corresponding to the free carboxylic acid in pABAH is diminished upon metal coordination in (I), Fig. 4. The strong bands at 1606 cm À 1 and 1404 cm À 1 correspond to the asymmetric (� asym ) and symmetric (� sym ) stretching vibrations of the carboxylate group of pABA in (I).The difference in the asymmetric and symmetric vibrations (�� = 202 cm À 1 ) corresponds to monodentate binding of the carboxylate which corroborates well with the structure of (I).
The stability of (I) was studied by thermal gravimetric analysis in the range of 30-500 � C, which shows that (I) is stable up to 300 � C. The initial loss of 2 wt% corresponds to the loss of coordinated water molecules, and the complete decomposition (94 wt%) corresponds to the evolution of CO 2 upon the decomposition of the carboxylate group in the ligand, pABA, leaving behind metal oxide ash (Fig. 5).The percentage of ash left behind is surprisingly lower than expected and might be due to the heterogeneity of the material.

Luminescence properties
The emission spectra of (I) and the pABA ligand were recorded at room temperature to assess the luminescence properties of the samples.For this, 1 mg of each sample was finely dispersed in 2 mL of water through ultrasonication.Their respective emission spectra were then recorded at an excitation wavelength of 280 nm, and excitation and emission slit widths of 1 and 1 nm, respectively, in the range 300 to 450 nm.It was found that the emission intensity of (I) is much more intense compared to the emission intensity of the pure pABAH ligand in water.Compound (I) also undergoes a slight blue shift of �� = 4 nm, which is representative of the binding of ligand (pABA) with the metal center (Cu II ) (Fig. 6).Li et al., 2009) as a threedimensional MOF.The carboxylic group of pABA is usually monodentate (Amiraslanov et al., 1978;Prondzinski & Merz, 2008), except in Cd II complexes ABZCUH (Amiraslanov et al., 1979b) and BESRAS (Turner, et al., 1982), where it is bidentate, and in RUPZIM where both mono-and bidentate coordination is present.Thus, compound (I) shows the most typical structural features, being a 1D coordination polymer with the pABA bridge coordinated via the amino group and one carboxylic O atom (Fig. 2b).

Refinement
Crystal data, data collection and structure refinement details are summarized in

Figure 2
Figure 2Binding modes of the pABA ligand in coordination polymers with Co II , Ni II , Cu II , Zn II or Cd II (shown as blue spheres).

Figure 1
Figure 1Tricarboxylic ligand (H 3 btta) used and its fragmentation to pABAH under hydrothermal reaction conditions.

Although
pABA is widely used as a ligand in the synthesis of coordination polymers and metal-organic frameworks, a survey of the Cambridge Structural Database (version 5.45, updated on 01/01/2024; Groom et al., 2016) revealed no Cu complexes containing only pABA ligands and coordinated or crystallization water, while such complexes are known for Co II , Ni II , Zn II and Cd II .Most of these are one-dimensional coordination polymers, although [Co(pABA)(H 2 O) 4 ] (ABZACO10; Amiraslanov et al., 1979a) crystallizes as discrete molecular units, [Zn(pABA) 2 (H 2 O)]•H 2 O (IWORET; Ibragimov et al., 2016) as a two-dimensional polymer, and [Zn(pABA) 2 ]•H 2 O (RUPZIM;

Figure 6
Figure 6Luminescence emission spectra of pure pABAH and (I) measured at room temperature in water (� excitation = 280 nm).

Table 2
Experimental details.

Table 2 .
The crystal studied was a merohedral twin with the twin components of equal size related by a 180 � rotation about the c axis.The water H atoms were refined in isotropic approximation, other H atoms as riding in idealized positions, with U iso (H) = 1.2�U eq of the bearing C or N atom.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes.Refinement.Refined as a 2-component twin.